Which alien worlds are most livable?

One of the several planets within the Gliese 581 star system, called Gliese 581d, ranks among the most potentially habitable alien worlds on a new scale.

Astronomers have come up with a livability index for alien planets and moons, and the winners are ... Titan in our own solar system, and the Gliese 581 planets in the extrasolar league.

Rating systems for Earthlike and habitable planets may not make much difference now, but the developers of the Earth Similarity Index and the Planet Habitability Index say they could be crucial in the years ahead.

"With a new generation of telescopes and missions on the way, the discovery of many more exoplanets can be expected," they write in a paper to be published in the December issue of the journal Astrobiology. "That, in turn, will drive the need for a classification scheme for assigning astrobiological potential for exoplanets based on estimates derived from quantitative data of their probability for supporting life."

If such a scheme could truly reflect whether or not a given planetary environment is habitable, that could drive the priorities for exploration in our own solar system, as well as high-resolution observations of extrasolar systems.

Habitability indexes have been in the works for at least the past couple of years. Traditionally, astrobiologists have focused on three conditions that appear essential for life on Earth: organic compounds, the presence of liquid water, and an energy source such as the sun or undersea volcanoes. But in the search for alien Earths, those conditions aren't easily determined, and they may even be irrelevant.

The newly proposed indexes take a two-track approach to the classification challenge.

"The first question is whether Earthlike conditions can be found on other worlds, since we know empirically that those conditions could harbor life," Dirk Schulze-Makuch, an astrobiologist at Washington State University who is one of the study authors, said in a news release. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."

The Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as the size and temperature of its parent star, and compares those parameters with Earth's. Earth has the maximum global ESI of 1. Mars has a 0.70 rating, and Mercury is the next on the list with 0.60. For what it's worth, the dwarf planet Pluto and Neptune's moon Triton register a measly 0.075 and 0.074, respectively. And Enceladus, the icy Saturnian moon that is thought to harbor a subsurface ocean and perhaps life, is right down there with them at 0.094.

Looking beyond the solar system, the researchers worked up ESI values for a variety of extrasolar planets. The top finishers were Gliese 581g (whose existence is in dispute) with 0.89, and Gliese 581d with 0.74.

But that's just the first part of the job: The researchers' Planet Habitability Index looks at a different set of factors: Does the planet have a rocky or frozen surface? Is there an atmosphere, and how thick is it? How about a magnetic field? How much energy is available, either through tidal flexing or from the parent star? Could there be organics present, and is a liquid solvent available for chemical interactions?

By those measures, Earth has a relative PHI of 0.96, which is nearly as close as you can get to the maximum of 1. Based on what's known about the rest of the solar system, the runner-up is not Mars, as you might expect, but the Saturnian moon Titan (0.64 vs. 0.59 for Mars). The Jovian moon Europa is next on the list (0.47), but Enceladus (0.35) ranks lower than Venus, Jupiter and Saturn (0.37).

The authors stress that expectations based on earthly life may not apply to extraterrestrial environments.

"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” they write. "For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life."

So how does the Gliese 581 system's PHI look? Gliese 581g's value was estimated at 0.45, 581d registered 0.43, and 581c came in at 0.41. By that scale, the chances of finding life in a red-dwarf system 20.5 light-years away (or sustaining life if we ever get there) are about as good as they are for Europa. OK, but not great.

It's important to keep a couple of things in mind about this research: First of all, there's a fair amount of speculation about the various factors and their relative value for habitability. Further observations may shift the values for those factors, as well as the mathematical formula into which they're fed.

Perhaps more importantly, the numbers game can't take the place of actual observation and exploration. The ESI and PHI may well turn out to be thought experiments like the Drake Equation, which takes your assumptions about a variety of cosmic factors (How many planets like Earth come into existence every year? How likely is it that intelligent civilizations arise on alien Earths? How long do they last?) and turns them into a number. At least that's the message from David Morrison, director of the Carl Sagan Center for the Study of Life in the Universe, headquartered at the SETI Institute in Mountain View, Calif.

Here's what Morrison told me in an email:

"Very interesting. Discussing such conceptual indexes is a good way to organize our thinking about worlds that may be suitable for life. But it doesn’t actually add value, in my opinion. For the Earth Similarity Index, we already have thought that liquid water, and a solid surface, and enough gravity to hold on to a substantial atmosphere, are important indications of habitability. Hence the interest in Earth-size planets within the habitable zone (meaning surface liquid water is possible). To go further, as by considering the composition of the atmosphere, we are quickly into the effort to identify life by its chemical signatures, not just habitability. The broader habitability index in also interesting, but we just don’t know how to define habitability. And if Titan is an example, we may never have the data on exoplanets that could distinguish the hydrocarbon liquid lakes that we see on Titan.

"Bottom line: This (like the Drake Equation) is a good teaching tool. It helps is to organize our thoughts. But I doubt it will be very useful as a research tool, because we know so little about what properties truly define habitability. Without a much better idea of what alien life is like, we don't know how to define habitability. And probably nature is much more creative than we can imagine."

What do you think? Where would you target the search for extraterrestrial life, and what criteria would you use to prioritize the targets? Feel free to weigh in with your comments below.